Scientists might be overestimating atmospheric cooling effects of aerosol pollution

Burning fossil fuels emits CO₂, warming our planet. But the microscopic aerosol particles emitted from tailpipes and smokestacks also seed clouds, which reflect sunlight and offset warming effects. Researchers now say current climate models might be overestimating this cooling phenomenon (Science 2021, DOI: 10.1126/science.abd3980).

To estimate the cooling contribution of aerosols emitted by humans, researchers have looked to ships. When a ship passes underneath the fluffy stratocumulus clouds that blanket large areas of the open ocean, the aerosol particles it emits rise into the clouds. Small water droplets form around the aerosols, creating bright white streaks of thicker cloud called ship tracks. By comparing the light reflected from ship tracks to that reflected by surrounding clouds, researchers can estimate the cooling attributable to these emitted particles.

Now Franziska Glassmeier of Delft University of Technology and colleagues say scientists need to rethink interpretations of ship track data, which has been extrapolated in climate models to estimate the contribution of emitted aerosols beyond the open ocean. The group’s simulations of cloud behavior found that while aerosol pollution at first thickens clouds and enhances cooling, persistent aerosol pollution, like from an industrialized region, can actually thin clouds; over time the smaller droplets that aerosols create evaporate faster, Glassmeier explains. The team’s estimates have high uncertainty, but the researchers think climate models may be overestimating the cooling effect of emitted aerosols by as much as threefold.

Climate and cloud scientists say the work has them rethinking some of their assumptions about aerosols. Nicolas Bellouin of the University of Reading says the work may lead climate modelers to slightly reduce aerosols’ cooling effects, although he points out that aerosol-cloud interactions remain a fluid area of research. Glassmeier and her colleagues don’t think scientists should abandon ship track data. They propose studying high-traffic shipping lanes as models of persistent pollution sources.